This invention relates generally to nonlinear optics. More particularly, it relates to noise suppression in optical parametric oscillators.
Optical Parametric Oscillators (OPOs) generally comprise a nonlinear material disposed within a resonant cavity. OPOs convert incident photons into photon pairs when optically excited at a power per unit area above a certain threshold. The threshold level is a characteristic of the nonlinear material and the resonator. A source such as a laser provides the incident photons in the form of pump radiation. OPOs and lasers are often subject to noise. Noise may arise in the pump source or in the current source that drives the pump source. If there is noise on the pump source there will also be noise on the resonated pump or signal within the OPO. Noise may also arise due to environmental acoustic sources or temperature fluctuations. Types of noise include white noise, 1/f noise and narrow band noise. Such noise is undesirable in many applications such as precision measurements, analog signal transmission and analog signal processing.
Prior art noise suppression techniques often depend on the frequency range in which the noise is to be suppressed. For example, high frequency noise may be suppressed with passive techniques such as mode cleaner cavities. Low frequency noise may be suppressed using active techniques, e.g. involving electronic feedback. Because a laser or an OPO may be subject to both low and high frequency noise, both types of noise suppression must often be used, which adds to the complexity and cost of the system.
OPO""s are usually embodied in one of two forms: Either a doubly resonant oscillator (DRO) in which both the generated optical beams are resonated or a singly resonant oscillator (SRO) in which only one of the generated optical beams is in resonance. In a pump resonant OPO (PROPO), the pump radiation resonates within the OPO cavity. A PROPO could be a singly resonant oscillator (SRO) or a doubly resonant oscillator (DRO). PROPOs are desirable because they can operate at a lower threshold than non-pump resonant OPOs. This also helps stabilize the OPO frequency. Furthermore, pump resonant OPOs are usually pumped with single frequency lasers, which are often very quiet.
One such quiet single frequency laser is the non-planar ring oscillator (NPRO). NPRO-based pump-resonant OPO""s have been demonstrated in several wavelength ranges. For examples the reader is referred to K. Schneider and S. Schiller, xe2x80x9cNarrow-linewidth, pump-enhanced singly-resonant parametric oscillator pumped at 532 nmxe2x80x9d, Applied Physics B 65, 775, (1997), and K. Schneider, P. Kramper, S. Schiller, and J. Mlynek, xe2x80x9cToward an Optical Synthesizer: A Single Frequency Parametric Oscillator Using PPLNxe2x80x9d, Opt. Lett. 22, 1293, (1997), and D. Chen, D. Hinkley, J. Pyo, J. Swenson, and R. Fields, xe2x80x9cSingle-Frequency, Low-threshold continuous-wave 3-xcexcm Periodically Poled Lithium Niobate Optical Parametric Oscillator.xe2x80x9d
A. E. Siegman, in xe2x80x9cNonlinear Optical Effects: An Optical Power Limiterxe2x80x9d, Appl. Opt. Vol 1, 739 (1962) proposed using an OPO to protect against large spikes in laser power, focusing on the large-dynamic-range aspects of the device. As a noise suppressor, the OPO described by Siegman would cut off output power sharply and flatly above a certain threshold. Siegman""s analysis a device which was not pump resonant. However, because the ignored pump depletion and gain saturation, Siegman concluded that an OPO was a perfect noise suppresser. In addition, Siegman""s analysis did not realistically address optimization of a design to minimize noise in an OPO under actual operating conditions. In effect, Siegman said that pump transmission in an OPO could not exceed the threshold value. As such, an OPO as described by Siegman would only cut off fluctuations of the resonant power level above the threshold, but would not suppress deviations of the resonant power below the threshold. Although later researchers demonstrated OPO operation with an incident pump power exceeding the threshold, none recognized doing noise suppression this way.
There is a need, therefore, for an improved Optical Parametric Oscillator with improved noise suppression.
Accordingly, it is a primary object of the present invention to provide an optical parametric oscillator (OPO) having increased suppression of noise at a wavelength of incident pump radiation. It is a further object of the invention to provide a method for optimizing noise suppression of pump radiation in an OPO. It is an additional object of the invention to provide an OPO with increased noise suppression at a wavelength of signal radiation.
These objects and advantages are attained by a pump resonant optical parametric oscillator (PROPO) operated above threshold for the purpose of noise suppression. The PROPO may be optimized for noise suppression of pump radiation of wavelength xcexp. The PROPO generally comprises a parametric amplifier disposed within a resonant cavity. The parametric amplifier has a gain G at a signal wavelength xcexs. The gain G increases monotonically with increasing power at the pump wavelength xcexp. The resonant cavity resonates at both pump wavelength xcexp and signal wavelength xcexs. The resonant cavity includes an input coupler having a transmission TIC at the pump wavelength xcexp. The cavity also includes at least one output coupler having a transmission TP at the pump wavelength. In addition the cavity may include another output coupler (possibly the same one) having a transmission Ts at the signal wavelength. The parametric amplifier transfers noise on the pump radiation to the signal radiation. TIC, TP, TS, and G are chosen such that a resonated pump power at xcexp is nearly clamped at a threshold level, such that a noise suppression of the pump signal is greater than about a factor of 10.
The noise suppression of the pump radiation may be optimized by an inventive method. According to the method Tp, TS and G are set such that the PROPO has a threshold slightly lower than the available power at a wavelength of the optical pump, thereby enabling noise on the pump radiation to be reduced by greater than about a factor of 10. TP is typically set to be within about a factor of two of a total passive loss xcex1P of the cavity. The cavity is typically impedance matched by setting TIC within about a factor of 2 of Tp+xcex1p.
An alternative embodiment of the invention provides a PROPO that is configured to suppress noise on a first signal radiation. The PROPO of the second embodiment generally comprises first and second parametric amplifiers disposed within a resonant cavity. The first parametric amplifier produces a resonant first signal radiation having a wavelength xcex1s and a first idler radiation in response to pump radiation at xcexp. The first signal radiation acts as a pump for the second parametric amplifier, whereby the second nonlinear crystal produces the second signal at wavelength xcex2s and a second idler. The cavity could resonate at both wavelength xcex1s and wavelength xcex2s. A second cavity containing the second nonlinear element could optionally be used to separately resonate the second signal. Alternatively, two coupled cavities may be used with at least one of the two nonlinear elements shared between the two cavities. One cavity resonates at xcex1s while the second cavity resonates at xcex2s. Noise on the first signal couples directly to the second signal and idler, thereby reducing noise on the first signal.